I looked in the manual but can't find it. While playing a pbem game How do you send a message to the other player? I looked but unable to find.

Second - When you look at other challenges - Is that what other players post and looking for games? I wonder b/c it seems more players would have challenges unless everyone does this through the forums.

Go into strategic map, click on your opponent's faction and then you should see a tab to send a message. And yes that is the open games showing in other challenges, I guess there is never many as once a challenge is accepted they no longer show up.